You said something far more alarming and also flat out wrong - that's what I'm refuting. This 2 or 10 or whatever tiny amount of birds at a time thing you are claiming to experience...doesn't match my experience whatsoever. I can provide more photographs if you don't believe me.
I can't speak to the pacific northwest 30 years ago so I'll take your word for it that there were more birds on the coast then.
Sorry you don't believe me.
Here's my own photo evidence from South Beach in July 2017. https://photos.app.goo.gl/JzporX8XmFFswgSu8
I didn't go in 2018, but 2016 and 2017 I was there for about a week. same empty beaches both trips.
You being there in Nov may be why you see seagulls, I never have been there in winter.
I don't have any photos handy from the 1980's to scan, but it truly is a stark difference.
Seagulls still exist obviously, but there numbers are drastically smaller. As they are basically rats with wings, perhaps it's just better trash management, but the observation still stands.